Silicon-germanium (SiGe) is a semiconductor compound consisting of covalently bonded silicon and germanium atoms. It is a substitution solid solution of the two elements that can infinitely dissolve into each other. SiGe heterojunction bipolar transistors (HBTs) manufactured by using SiGe materials have become a kind of commonly used radio frequency (RF) devices.
Chinese patent application Number 201110370460.X, entitled “Ultra-high Voltage SiGe HBT and Manufacturing Method Thereof”, filed on Nov. 21, 2011, claims an ultra-high voltage SiGe HBT, as schematically illustrated in FIG. 1.
In a substrate 101, there are formed two isolation structures 102, two pseudo buried layers 103 and a collector region 104. The isolation structures 102 are formed by etching the substrate 101 to form trenches therein and filling a dielectric material into the trenches. The pseudo buried layers 103 are two doped regions each formed under a corresponding isolation structure 102. The pseudo buried layers 103 are formed by implanting ions into the bottoms of the trenches with a high dose and a low energy, and characterized in a shallow junction and a high dopant concentration. The collector region 104 is a doped region located between the two isolation structures 102 and between the two pseudo buried layers 103 as well. It is formed by implanting ions into a portion of the substrate 101 that is sandwiched between the isolation structures 102. Additionally, the collector region 104 has a depth greater than that of the isolation structure 102 and is in contact with the pseudo buried layer 103 on each side of it. A SiGe base region 105 is formed on the collector region 104. The SiGe base region 105 has its two ends above the respective isolation structures 102 that are adjacent to the collector region 104 or has its two ends both situated above the collector region 104. A SiGe field plate 106 is formed on each of the isolation structures 102, and is located right above a border of the collector region 104 and a corresponding one of the pseudo buried layers 103. Moreover, both the SiGe base region 105 and the SiGe field plates 106 are formed by growing a SiGe epitaxial layer and etching it. Dielectric layers 107 and a polysilicon emitter region 108 are formed above the SiGe base region 105. The polysilicon emitter region 108 has a T-shaped vertical cross section (i.e., broader at the top and narrower at the bottom) and is in contact with the SiGe base region 105 at the bottom. The dielectric layers 107 are formed between the SiGe base region 105 and extending portions of the polysilicon emitter region 108. Each side face of the SiGe base region 105 and each side face of each SiGe field plate 106 is covered with a first sidewall 109. In addition, each side face of the polysilicon emitter region 108 is covered with a second sidewall 110. A first electrode 111 is formed through an interlayer dielectric (ILD) layer as well as a corresponding isolation structure 102 and is in contact with a corresponding one of the pseudo buried layers 103. Moreover, second electrodes 112, third electrodes 113 and a fourth electrode 114 are formed through the ILD layer and are in contact with a corresponding one of the SiGe field plates 106, the SiGe base region 105 and the polysilicon emitter region 108. Furthermore, each first electrode 111 is interconnected with a corresponding second electrode 112, and they jointly serve as a collector. Each third electrode 113 serves as a base, and the fourth electrode 114 serves as an emitter.
In this ultra-high voltage SiGe HBT, each pair of a pseudo buried layer 103 and a corresponding first electrode 111 serves as a structure for picking up the collector region. A base-collector (BC) junction (i.e., the PN junction between the SiGe base region 105 and the collector region 104) is characterized in a two-dimensional potential distribution. That is, the BC junction has not only a vertical extension in the downward direction from the SiGe base region 105 to the substrate 101, but also horizontal extensions in the lateral directions from the SiGe base region 105 to the respective pseudo buried layers 103. This feature improves the breakdown voltage in common-emitter configuration, Bvceo, of the device up to 5 volts to 20 volts. For this reason, the device is referred to as an “ultra-high voltage” SiGe HBT device.
Nevertheless, this device has a drawback that, as the two heavily-doped pseudo buried layers 103 are separated from each other by a rather great distance, the lightly-doped collector region 104 that is connected to both the pseudo buried layers will accordingly have a great width. With a determined doping concentration of the collector region 104, the great width typically leads to a high series resistance of the collector region 104 and a great saturation voltage drop of the device. Therefore, the device has a small linear region which limits its applications. On the other hand, although lowering the dopant concentration of the collector region 104 can lead to reduction of the series resistance as well as the saturation voltage drop, it will also lead to reduction of the device's breakdown voltage.